ristiluu
Ristiluu, known in English as the sacrum, is a large triangular bone at the base of the spine and a key part of the pelvic girdle. It is formed by the fusion of five sacral vertebrae (S1–S5) during adulthood. The base of the sacrum articulates with the fifth lumbar vertebra (L5) at the lumbosacral joint, while its lateral parts, the ala (wings), join the ilia at the sacroiliac joints. The apex of the sacrum meets the coccyx.
The sacrum has both anterior and posterior surfaces with openings that transmit nerves and vessels. The anterior
Functionally, the sacrum transmits body weight from the spine to the pelvis, stabilizes the pelvis, and helps
Development and variation: typically, the sacral vertebrae fuse by adulthood. Variation can include sacralization of L5
Clinical relevance: sacral fractures, usually from high-energy trauma, can affect the sacral nerves and pelvic stability.